We have determined the structure of the Light-Harvesting Complex-II (LH-II) of Rhodospirillum molischianum to 2.4 A resolutionby the molecular replacement method using a computationallymodelled probe structure, in collaboration with Hartmut Michelat the Max-Planck-Institut, Frankfurt, Germany [16, 9].LH-IIis part of the photosynthetic apparatus of purple bacteria. Themain function of the light-harvesting complex II is to gather light energy and to transfer this energy to the photosynthetic reaction centers for photosynthesis. The LH-II of Rhodospirillummolischianum is an octameric aggregate of a basic unit consisting of a pair of polypeptides, commonly referred to as the ff- and the fi-apoprotein; each basic unit noncovalently binds three bacteriochlorophyll-a and one lycopene molecule.The crystal structure displays two concentric cylinders of sixteen membrane-spanning helical subunits which provide the framework to supporttwo rings of BChl-a molecules, one formed of sixteen B850 BChl-a's perpendicular to the membrane plane and the other of eightB800 BChl-a's nearly parallel to the membrane plane. This workrepresents the first successful attempt at using ab initio structureprediction to solve the phase problem in X-ray crystallographic structure determination [16, 9].The approach promises to further extend the applicability of X-ray diffraction methods in thedetermination of biomolecular structures. All simulations were carried out with the HP workstation cluster at the Resource using X-PLOR and VMD.